Luminescent tube



June 9, 1931. TEASDALE 1,808,825

LUMINESCENT TUBE Filed May 4, 1929 /0 cluuunnnuuu, v." /3 I 4 88140411417010: George E Teasdale,

Wines? Patented June 9, 1931 GEORGE 'E. TEASDALE, KANSAS CITY, KANSAS VLuMINEseEN'r TUBE Application and May 4, 1929. Serial No. 360,446.

My invention relates to illumination by luminous tubes of glasscontaining rare atmospheric gases, such, for lnstance, as neon gas WhlChbecomes lumlnescent by the passage therethrough of high voltage electriccurrents introduced into the gas through electrodes located in the endsof the tube. The electrodes are in direct contact withthe gas andconnected to the inner ends of terminal wires which extend outwardlythrough the ends of the tube and are connected to the two sides of ahigh voltage circuit.

' When the high voltage isapplied to the electrodes, the neon gas inthetube acts as a conductor a nd becomes brilliantly illuminated, givingoff a bright orange red color which makes it very desirable foradvertising and other commercial purposes, especially when the tubes areformed into let- 'ters, numbers and various other designs.

Difficulty in the manufacture of the tubes has been experienced becauseregardless of how pure the. gas was when introduced into the tube, whenilluminatedfit became badly discolored from impurities thrown off by theelectrodes and the impure active gases present in the air within thetube. In order to overcome the foregoing difficulty the air existing inthe tube prior to the introduction of the neon was exhausted to a highstate of vacuum, but in spite of the precautions which were taken tointroduce extremely pure neon into the-tubes, and of the care taken inexhausting the air from said tubes, nevertheless, impurities wouldappear when the neon was illuminated as the electrodes. and the glasswalls of the tubes, absorband hold on their exposed surfaces im-,purities such as the active foreign gases, and when heat is applied tothose surfaces they yield up these foreign gases, which are known in theart as occluded gases. It was found necessary to remove enough of thesegases from the electrodes and walls of the tube by subjecting the latterto a'current in excess of normal so that when the tubes are placed incommercial use no more occluded gases will be driven out of these partsdue to such heating as takes place during normal operation. I

It was also discoveredthat notwithstanding the liberation of the'activegases occludedin the electrodes and walls during the formation of thetubes, impurities were still resent after the neon was introduced.

hese gases have hitherto been removed by attaching to the tube'areceptacle containing powdered cocoanut'charcoal which, when reduced toan extremely low temperature by immersing the receptacle in liquid air,absorbs the impure gases and leaves the neon in an approximately purestate.

After the neon in the tube had thus been purified it was found thatthere was an other serious difficulty in the way, for although the tubehad attained the characteristic neon color, its life was relativelyshort compared with that of an incandescent lamp which averaged onethousand hours, whereas it was desired to produce a neon tube in whichone introduction of neon gas would yield a life much longer than that ofthe incandescent lamp.

Experimentation developed the fact that the short life of the tube wasattributable to a phenomenon connected with the electrodes known asvaporization or sputtering. In the presence of the rare gases such asneon, the sputtering of the electrodes was very pronounced particularlywhen the electrodes had a surface area of less than 1.5 squaredecimeters per ampere, said sputtering being caused by a drop inpotential or voltage from the electrodes to' the point where theluminous part of the gas appears and causes the electrodes to becomehighly heated. The high temperature causes the electrodes to sputter atahigh rate so that particles of material composin the electrode/arethrown off and deposlted on the glass'walls of the tube in which theelectrodes are located. As the particles of material are thrown off theycarry and pocket a certain quantity of the neon gas against the glasswalls. This pocketing causes so much of the gas to be used up and itspressure so reduced, that it no longer remains conductive and the lampbecomes extinguished.

, It is the object of my invention to overcome or reduce to a minimumthe foregoing difiiculties encountered in the manufacture and use of thetubes and the electrodes, to the end that'they may be more quickly andeconomically produced in large quantities and will last indefinitely.

In the accompanying drawings: Fig. 1

- shows one of the tubes in the course of construction.

'vided at each of its enlarged ends with an inwardly projecting stem 4,the inner eonvex end of which may be slightly enlarged as indicated at 6for the reception of an electrode 8 consisting preferably of platinumand the back of which is sealed in said enlarged end 6 during theformation of the latter. The electrodes 8 are of concave-convex form toadd rigidity thereto and in order that they may snugly fit the convexends of the respective stems 4. Wires 10 lead outwardly to a suitablesource, not shown from the respective electrodes 8 through the stems 4in which said wires are hermetically sealed.

7 During the formation of the tube 2 it is provided with a receptacle12, to the neck 13 of which a pipe 14 is connected and equipped with avalve 16 and a hose 18, which latter is adapted to be'connected to avacuum pump not shown. A branch 20 through which the gas neon isadmitted is connected to the pipe 14 and provided with a valve 22 forcontrolling the flow of the neon into the tube 2.

The tube 2 is charged with neon through which an electric current ofapproximately normal strength is'transmitted for expelling the foreigngases from the glass walls and the electrodes. The tube is alsosubjected to an external temperature ofapproximatcly 400 degrees F. tohasten the expulsion of the gases which are then removed from the tubethrough the action of the vacuum pump.

Any trace of foreign gases admitted with the neon to the tube 2 areabsorbed from the latter by a suitable chemical such as anhydrousphosphorous pentaoxide with which the receptacle 12-issupplied. The tube2 is then sealed where it jolns the neck 13 and is separated from thereceptacle 12, after which it is ready for use.

The current used for liberating the oceluded gases from the platinumelectrodes 60' need not be so great as that used on other electrodes ofless density because when heated toapproximately 130 degrees C. theplatinum is transformed"into a catalytic agent, causing the combinationof carbon monoxide with oxygen to'form carbon dioxide, which gases arereadily removed through the operation of the vacuum 'pump and theapplication of external heat of approximately 400 degrees F. Neither isit necessary to prolong the treatment of the platinum electrode as withcopper and other electrodes, as-the exposed area of the platinumelectrode is considerably smaller and gives off only about one-tenth ofits volume in occluded gases, whereas copper gives off about one-sixthof itsvolume in occluded gases when subjected to the same conditions.

When made of platinum the electrodes 8 need not have an exposed area ofmore than 1 square decimeter per ampere, because I have found thatplatinum is far superior to copper, carbon and other materialsheretofore used for electrodes as the platinum.

being less porous contains less of the detrimental gases and does notrequire the prolonged treatment to whichthe other electrodes, must besubjected for the purpose of eliminating the impure gases. Further more,the exposed area of the platinum electrode need be but a small fractionof that of other electrodes heretofore employed, thereby furtherreducing the volume of foreign gases to be eliminated. Also, owing tothe density of the platinum less vaporization takes place during thepassage of an electric current and it will not be necessary torepeatedly admit neon to the tube for the purpose of scavenging thelatter, as with other methods.

By eliminating the practice of using repeated charges of neon whenscavenging the tubes during their formation a material saving of thisexpensive gas is effected. A further saving in the cost of manufactureis effected by employing anhydrous phosphorous pentaoxide for absorbingthe foreign gases from the neon on the spot instead of usin cocoanutcharcoal in a receptacle which is immersed in liquid air to reduce thetemperatureof the charcoal to a low enough de ree to render it effectivein absorbing the oreign gasesQ Having thus described my invention, whatI claim and desire to secure by Letters Patent, is:

In combination, a glass tube closed at each end, a solid stem formedintegral with each end of said tube'and extending into the latter, eachstem having'aconvex terminal, a concavo-convex'electrode fixed to theinner end of each stem, and a conductor connected to each electrode. 7

In testimony whereof -I afiix my signature.

GEORGE E. TEASDALE.

